As a gas sensor which is installed in the exhaust system of an internal combustion engine and used for combustion control of the internal combustion engine by detecting the concentration of oxygen in exhaust gas, conventionally, an oxygen sensor has been well known. This oxygen sensor, for example, comprises a cylindrical main body metal and a sheet-like gas sensing device held by the main body metal. The gas sensoring device comprises a first solid electrolyte layer extending in the length direction, a cell having a first opposing electrode formed on front and rear surfaces on the side of the front end exposed to measuring object gas of the first solid electrolyte layer and a first porous portion overlaid on the cell. One of the opposing electrodes of the cell is disposed in a measuring chamber to which measuring object gas is to be introduced. The first porous portion is provided to control the diffusion rate of the measuring object gas introduced into the measuring chamber.
Some types of fuel and engine oil used in the internal combustion engine of an automobile or the like contain phosphorous or silicone. When this fuel or engine oil is used, phosphorous or silicone adheres to the surface of the first porous portion so as to close pores in the porous portion so that the first porous portion is clogged. As a result, the diffusion resistance of the first porous portion changes, so that the detection accuracy of the air-fuel ratio of the gas sensor can drop.
To meet generation of clogging in the porous portion and abnormality in the electrode, providing of a second porous portion for preventing poisoning by phosphorus or silicone between the first porous portion and outside has been known. See, for example, Japanese Paten Application Laid-Open No. 10-221304 to Tsuzuki et al. and U.S. Pat. No. 5,925,814 to Tsuzuki et al. Generation of clogging in the first porous portion can be suppressed by sucking phosphorous or silicone with the second porous portion. Thus, changes in diffusion resistance in the first porous portion can be suppressed to block drop of the detection accuracy of the air-fuel ratio of the gas sensor.
However, in recent years, higher performance and intensified accuracy of the gas sensor have been demanded and thus, suppressing of clogging generated in the first porous portion by sucking more phosphorous and silicone by means of the second porous portion has been considered important. However, there is a fear that the second porous portion described in the aforementioned patent documents cannot suck phosphorus and silicone sufficiently enough for higher performance and intensified accuracy of the gas sensor. More specifically, the second porous portion of the aforementioned patent documents is so constructed that all measuring object gas is introduced into an introduction passage in order to introduce to a measuring chamber and an interface between the introduction passage and the second porous portion is exposed on an external face of the gas sensing device. In case of such a gas sensing device, when the measuring object gas is introduced into the second porous portion, part of phosphorous and silicone invades into the first porous portion from outside using this interface as a passage without being sucked by the second porous portion. As a result, suppression of clogging generated in the first porous portion is not achieved sufficiently, so that there is a fear that the first porous portion cannot be applied to intensification of performance and accuracy of the gas sensor.